1. Field of the Invention
This invention relates generally to valve lifters for combustion engines. More specifically, this invention relates to an improved lifter composition and method of fabricating a lifter for aircraft and other combustion engines, for reducing lifter damage or failure due to thermal fatigue and/or wear.
2. Related Art
Lifters 10 for combustion engine valve systems 12 serve as the contact members for the rotating cams 14. During each rotation of the cam 14, the cam surface 16 rotates into contact with the lifter 10 and slides along the lifter's face surface, thus pushing the lifter 10 with a force having a component parallel to the longitudinal axis of the lifter. The cam's rotating motion is therefore translated to linear motion of the lifter and the connected push rod 18 and, hence, to a pivoting motion of the rocker arm 20, in order to control the valve position 21.
The repeated, rapid engagement and disengagement of the cam surface 16 with the lifter face surface 22 presents a great potential for wear and/or damage to the lifter. Typically, there is a small clearance of several thousandths of an inch between the cam 14 and the lifter 10 when the cam is rotated so that the small diameter surface 17 faces the lifter. This clearance is designed to allow for heat expansion. Therefore, when the large-diameter portion 24 of the cam rotates toward the lifter 10, portion 24 hits the lifter and the cam surface 16 slides along the lifter surface 22 to raise the lifter 10. Then, as the small-diameter portion 26 rotates toward the lifter, the cam lowers the lifter and disengages from the lifter 10 due to the clearance. Thus, in each full rotation, the lifter and cam surfaces leave contact with each other during part of the cam's rotation, and then hit each other again for the remainder of the rotation.
Because of this demanding environment, the lifter has often been a point of wear and fatigue in the valve system 12, resulting in frequent change-outs of the lifters 12, or, worse, in engine damage. For many aviation engines, a major maintenance program is the replacement of lifters on a frequent and set schedule to prevent possible engine damage and/or failure.
Conventional valve lifters typically are generally cylindrical and solid, and are typically composed of cast iron, cast iron alloys, or other ferrous materials. Many of today's aircraft lifters are made from low grade steel that has been heat-treated to harden their face surfaces. The hardened, low grade steel surface tends to crack under the stresses of temperature changes and the repeated impact of the cam, and the cracked lifter then grinds down the cam surface, resulting in cam/lifter failure or engine failure.
Two-piece lifter bodies have been made, for example, as in U.S. Pat. No. 3,200,801, wherein a portion of low alloy steel is interposed between a stainless steel portion and an alloy cast iron foot piece. U.S. Pat. No. 3,657,800 discloses a lifter wear plate of graphitic alloy steel friction welded to a steel tube. U.S. Pat. No. 4,251,273 discloses various methods of manufacturing lifters.
What is still needed is a strong and durable valve lifter that may be made of economical materials. What is needed is a valve lifter that requires less frequent replacement and causes less frequent failure.